Filter apparatus

ABSTRACT

A filter apparatus in accordance with the present invention comprises, for example, a band-pass filter and a plurality of band-stop filters connected as the post-stage to the band-pass filter. The band-pass filter transmits a band containing a desired frequency band, and the band-stop filters attenuate undesirable frequency bands. Another filter apparatus in accordance with the present invention comprises, for example, a first band-pass filter, a circulator connected to the first band-pass filter, and second band-pass filters connected to the circulator. The first band-pass filter transmits a band containing a desired frequency band to the circulator, and the second band-pass filters transmit undesirable frequency bands and reflect the desire frequency band to the circulator. The desired frequency band is outputted from this circulator. Still another filter apparatus in accordance with the present invention comprises, for example, a circulator and a band-pass filter connected to the circulator. The band-pass filter transmits an undesirable frequency band and reflects a desired frequency band to the circulator. The desired frequency band is outputted from this circulator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a filter apparatus, and specifically,for example, relates to a filter apparatus for use in a cellulartelephone of and other radio equipment.

2. Description of the Prior Art

In recent years, in the United States, following a rapid propagation andexpansion of cellular car telephones and a rapid increase in the numberof cellular base stations, extension of the width of frequency bands tobe used has been studied to increase the number of channels to be used,and assignments of frequency bands to be used have been announced. Inthis situation, the frequency to be used in a service system sometimesapproaches the frequency to be used in another service system, andtherefore, to prevent mutual interference between those service systems,for example, a filter apparatus having a sharp characteristic as shownin FIG. 14 and a filter apparatus having a sharp characteristic as shownin FIG. 15 are requested.

To obtain such a sharp characteristic, it has been considered toconfigure a filter apparatus using a cavity resonator, but thisapparatus becomes large in shape, going against the request forminiaturization.

Also, conventionally, in a filter apparatus using resonators, to obtainsuch a sharp characteristic, the value of Q is required to be increasedby coupling each active circuit to each resonator respectively,resulting in a complicated structure.

Furthermore, in the conventional filter using resonators, a load isconnected to the output end thereof, and therefore the value of Q₀ ofthe resonator has to be made considerably large taking the effect of theload into account. In such a case, an active circuit coupled to theresonator has to be operated in an unstable state, and thereby theapparatus lacks practical usability.

SUMMARY OF THE INVENTION

Therefore, a principal object of the present invention is to provide afilter apparatus which is small in size and has a sharp characteristic.

Also, another object of the present invention is to provide a filterapparatus which can obtain a sharp characteristic by a simple structure.

A first filter apparatus for solving the principal object of the presentinvention is a filter apparatus wherein a desired frequency band islocated in a pass band thereof and undesirable frequency bands arelocated in attenuation bands thereof, and comprises an input terminal, aband-pass filter an input end of which is connected to the inputterminal and which is for transmitting a band containing theabove-mentioned pass band, a plurality of band-stop filters which areconnected in series as the post-stage to this band-pass filter and arefor attenuating the above-mentioned attenuation bands, and an outputterminal connected as the post-stage to a plurality of the band-stopfilters, and the band-stop filter comprises a resonator and an activecircuit which is coupled to the resonator and becomes a negativeresistance in an operation band of the band-stop filter.

In this first filter apparatus, the band-pass filter transmits a signalof the band containing the desired frequency band. Then, a plurality ofthe band-stop filters attenuate signals of the undesirable frequencybands. In this case, the active circuit coupled to the resonator in theband-stop filter becomes a negative resistance, and therefore the valueof Q of the band-stop filter becomes large. For this reason, theplurality of the band-stop filters sharply attenuate the signals of theundesirable frequency bands.

A second filter apparatus for solving the principal object of thepresent invention is a filter apparatus wherein a desired frequency bandis located in a pass band thereof and undesirable frequency bands arelocated in attenuation bands thereof, and comprises an input terminal, afirst band-pass filter an input end of which is connected to the inputterminal and which is for transmitting a band containing theabove-mentioned pass band, a non-reversible circuit element a firstterminal of which is connected to an output end of the first band-passfilter, a plurality of second band-pass filters input ends of which areconnected in common to a second terminal of the non-reversible circuitelement which becomes the output side when the first terminal is theinput side and pass bands of which are located in the abovementionedattenuation bands, and an output terminal which is connected to a thirdterminal of the non-reversible circuit element which becomes the outputside when the abovementioned second terminal is the input side, and thesecond band-pass filter comprises a resonator and an active circuitwhich is coupled to the resonator and becomes a negative resistance inan operation band of the second band-pass filter.

In this second filter apparatus, the first band-pass filter transmits asignal of the band containing the desired frequency band, and thenon-reversible circuit element gives the signal to a plurality of thesecond band-pass filters. The second band-pass filters transmit signalsof the undesirable frequency bands, and reflect the signal of thedesired frequency band of the non-reversible circuit element. In thiscase, the active circuit coupled to the resonator in the secondband-pass filter becomes a negative resistance, and therefore the valueof Q of the second band-pass filter becomes large. For this reason, aplurality of the second band-pass filters sharply transmit theundesirable frequency bands, and sharply reflect the desired frequencyband. Then, the non-reversible circuit element gives the reflecteddesired frequency band to the output terminal.

A third filter apparatus for solving another object of the presentinvention is a filter apparatus wherein a desired frequency band islocated in an attenuation band thereof, and comprises an input terminal,a non-reversible circuit element a first terminal of which is connectedto the input terminal, a band-pass filter which has a resonator an inputend of which is connected to a second terminal of the non-reversiblecircuit element which becomes the output side when the first terminal isthe input side and a pass band thereof is located in the above-mentionedattenuation band, and an output terminal which is connected to a thirdterminal of the non-reversible circuit element which becomes the outputside when the above-mentioned second terminal is the input side.

In this third filter apparatus, when a signal is inputted to the inputterminal, the non-reversible circuit element gives the signal to theband-pass filter. The band-pass filter transmits a signal of theattenuation band, and reflects signals of the bands other than theattenuation band to the non-reversible circuit element. In this case,the value of Q of the band-pass filter becomes large by coupling theactive circuit becoming a negative resistance in the operation band ofthis band-pass filter to any one of the resonators thereof, and theband-pass filter sharply transmits the attenuation band, and sharplyreflects the bands other than the attenuation band. Then, thenon-reversible circuit element gives the signals of the reflected bandsto the output terminal.

In accordance with the present invention, a filter apparatus having asharp characteristic can be obtained. Moreover, this filter apparatususes the resonator in place of the cavity resonator, therefore beingsmall in size.

Also, in accordance with the present invention, a filter apparatus iscontainable which can obtain a sharp characteristic by a simplestructure that the active circuit is coupled to any one of theresonators of the band-pass filter. Furthermore, in this filterapparatus, no load is required to be connected to the output end of theband-pass filter, and therefore the value of Q₀ of the resonator in theband-pass filter is not required to be so large, and the active circuitcoupled to the resonator of the band-pass filter can be operated instable state, and thereby the apparatus excels in practical usability.

The above and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the disclosed embodiments of the present invention whentaken in conjunction with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration view showing an embodiment of the presentinvention.

FIG. 2A is an illustration view showing a major part of a band-stopfilter used for the embodiment of FIG. 1, and FIG. 2B is an equivalentcircuit diagram of the major part of the same.

FIG. 3 is an equivalent circuit diagram of the band-stop filter used forthe embodiment of FIG. 1.

FIG. 4 is a graph showing a frequency characteristic of the band-stopfilter as shown in FIG. 3.

FIG. 5 is a graph showing a frequency characteristic of the embodimentof FIG. 1.

FIG. 6A is an illustration view showing a major part of another exampleof the band-stop filter used for the embodiment of FIG. 1, and

FIG. 6B is an equivalent circuit diagram of the major part thereof.

FIG. 7 is illustration view showing another embodiment of the presentinvention.

FIG. 8A is an illustration view showing a major part of a secondband-pass filter used for the embodiment of FIG. 7, and

FIG. 8B is an equivalent circuit diagram of the major part of the same.

FIG. 9 is an equivalent circuit diagram of the second band-pass filterused for the embodiment of FIG. 7.

FIG. 10 is a graph showing a frequency characteristic of reflection atthe input end side of the second band-pass filter as shown in FIG. 9.

FIG. 11 is a graph showing a frequency characteristic of the embodimentof FIG. 7.

FIG. 12 is an illustration view showing a major part of another exampleof the second band-pass filter used for the embodiment of FIG. 7.

FIG. 13 is an equivalent circuit diagram of the second band-pass filteras shown in FIG. 12.

FIG. 14 and FIG. 15 are graphs showing frequency characteristics whichare the background of the present invention and are required for it.

FIG. 16 is an illustration view showing a further embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an illustration view showing an embodiment of the presentinvention. This filter apparatus 10 comprises, for example, a box-shapedcase 12 composed of metal. For example, a coaxial connector 14 is fixedto one side wall of this case 12 as an input terminal in a manner ofpenetrating therethrough.

Also, a first band-pass filter 16 comprising, for example, dielectriccoaxial resonators is housed in this case 12. This band-pass filter 16has a pass band in a band (about 820-853 MHz in this embodiment)containing desired frequency bands (a band of 835-845 MHz and a band of846.5 -849 MHz in this embodiment). Accordingly, this band-pass filter16 transmits a signal of the band containing the desired frequencybands. An input end 16a of this band-pass filter 16 is connected to thecoaxial connector 14 as an input terminal, for example, by a coaxialcable.

Furthermore, for example, three band-stop filters 18, 20 and 22 arehoused in the case 12. These band-stop filters 18, 20 and 22 are forattenuating undesirable frequency bands, that is, a band of 845-846.5MHz, a band of 849 MHz and higher, and a band of 835 MHz and lower amongbands transmitted by the band-pass filter 16 respectively, and areconnected in series as the post-stage to the band-pass filter 16. Thismeans that an output end 16b of the band-pass filter 16 is connected toan input end 18a of the first-stage band-stop filter 18, an output end18b of the first-stage band-stop filter 18 is connected to an input end20a of the intermediate-stage band-stop filter 20, and an output end 20bof the intermediate-stage band-stop filer 20 is connected to an inputend 22a of the last-stage band-stop filter 22 respectively, for example,by coaxial cables.

These band-stop filters 18, 20 and 22 differ in the number of stages ofthe dielectric coaxial resonator to be used and the band to beattenuated, but have structures similar to one another, and therefore adetailed description will be made particularly of on the first-stageband-stop filter 18.

This first-stage band-stop filter 18 comprises, for example, acuboid-shaped case 24 composed of metal, and, for example, space in thiscase 24 is divided into eight housing parts by seven partition plates 26composed of metal. Then, in each housing part, a dielectric coaxialresonator 28 of, for example, λ/4 and a substrate 30 composed of aninsulating material, for example, epoxy resin are housed. In this case,an outer conductor 28a as one end of each dielectric coaxial resonator28 is grounded to the case 24. Also, each substrate 30 is disposed atthe open end side of each dielectric coaxial resonator 28.

As shown in FIG. 2A, on this substrate 30, for example, an L-shapedconductor pattern 32 is formed, and to one end of this conductor pattern32, an inner conductor 28b as the other end of the dielectric coaxialresonator 28 is connected, for example, by a conductor ribbon 34.Furthermore, on the substrate 30, two electrodes 36a and 36b areinstalled which produce gap capacities in cooperation with the conductorpattern 32. To these electrodes 36a and 36b, an amplifier circuit 40,namely an active circuit which is configured, for example, with a Sibipolar transistor and so on is connected by lines 38a and 38b for phaseadjustment. Accordingly, this amplifier circuit 40 is coupled to thedielectric coaxial resonator 28. Also, this amplifier circuit 40 is soconfigured as to become a negative resistance in an operation band ofthe band-stop filter 18. For this reason, the apparent value of Q ofthis band-stop filter 18 becomes large in the operation band of theband-stop filter 18. Furthermore, on the substrate 30, a conductorpattern 42 for supplying power to this amplifier circuit 40 is formed.

On the other hand, to the other end of the abovedescribed conductorpattern 32, for example, one end of a chip-shaped capacitor 44 issoldered, and the other end of the capacitor 44 is soldered to a centerconductor 46a of a coaxial cable 46 as a transmission line. Accordingly,an equivalent circuit of the dielectric coaxial resonator 28 andsurrounding components is formed as shown in FIG. 2B. The dielectriccoaxial resonator 28 and capacitor 44 are selected in the band of845-846.5 MHz to be attenuated.

Then, in this band-stop filter 18, the eight stages each comprising thedielectric coaxial resonator 28, the amplifier circuit 40 and the likeare connected by the above-described coaxial cable 46. Furthermore, theends of that coaxial cable 46 are connected to the input end 18a and theoutput end 18b which penetrate through one side wall of the case 24,respectively. Accordingly, an equivalent circuit of this band-stopfilter 18 is formed as shown in FIG. 3. Also, this band-stop filter 18has a frequency characteristic as shown in FIG. 4.

In addition, though not illustrated, the conductor pattern 42 of eachsubstrate 30 is connected to a power source end 18c penetrating throughthe other side wall of the case 24. Then, to this power source end 18c,a DC power source 48 in the case 12 is connected, and power is suppliedfrom the DC power source 48.

The intermediate-stage band-stop filter 20 has a structure similar tothe above-described band-stop filter 18, but differs therefrom in thatit has 10-stages each having a dielectric coaxial resonator and the likeand in that the attenuation band is the band of 849 MHz and higher.

Also, the last-stage band-stop filter 22 differs from theabove-described band-stop filter 18 in that it has 18 stages each havinga dielectric coaxial resonator and the like and in that the attenuationband is the band of 835 MHz and lower.

Power is supplied from the DC power source 48 also to power source ends20c and 22c of these band-stop filters 20 and 22.

Furthermore, the output end of the last-stage band-stop filter 22 isconnected to a coaxial connector 50 as an output end penetrating throughthe side wall of the case 12.

In this filter apparatus 10, the band-pass filter 16 transmits a signalof the band containing the desired frequency bands, and the post-stageband-stop filters 18, 20 and 22 sharply attenuate signals of theundesirable frequency bands among signals transmitted by the band-passfilter 16, that is, signals of the band of 845-846.5 MHz, the band of849 MHz and higher and the band of 835 MHz and lower, respectively.Accordingly, this filter apparatus 10 has a frequency characteristic asshown in FIG. 5.

In addition, in the above-described embodiment, dielectric coaxialresonators of λ/4 are used in the band-stop filter, but dielectriccoaxial resonator of λ/2 may be used in place of λ/4. In this case, forexample, as shown in FIG. 6A and FIG. 6B, one end side of the innerconductor 28b of the dielectric coaxial resonator 28 of λ/2 is coupledto the amplifier 40 as an active circuit, and further, for example, ametal cylindrical body 43 is fixed to the other end side of the innerconductor 28b with a conductive adhesive, and the both ends of thecapacitor 44 are soldered to the metal cylindrical body 43 and thecoaxial cable 46 as a transmission line. Or, for the resonator used inthe band-stop filter, a resonator having another structure such as astrip-line resonator may be used, without being limited to thedielectric coaxial resonator, and the resonator mode thereof may be amode such as the TE mode or the TM mode.

FIG. 7 is an illustration view showing another embodiment of the presentinvention. This embodiment differs from the embodiment as shown in FIG.1 particularly in the construction of the post-stage following the firstband-pass filter 16.

This means that in this embodiment, the output end 16b of the firstband-pass filter 16 is connected to a first terminal 19a of a circulator19 which may be referred to more broadly as a non-reversible circuitelement.

Furthermore, input ends 21a, 23a and 25a of three second band-passfilters 21, 23 and 25 are connected in common to a second terminal 19bwhich becomes the output side of the circulator 19 when the terminal 19aof this circulator 19 is the input side.

These second band-pass filters 21, 23 and 25 have pass bands in bands tobe attenuated among bands transmitted by the band-pass filter 16, forexample, a band of 845-846.5 MHz, a band of 849 MHz and higher and aband of 835 MHz and lower, respectively. These second band-pass filters21, 23 and 25 differ in the number of stages of the dielectric coaxialresonator to be used or the like and the pass band, but have structuressimilar to one another, and therefore a detailed description will bemade particularly of one of the second band-pass filters 21.

Like the band-stop filter 18 in the embodiment as shown in FIG. 1, thissecond band-pass filter 21 comprises eight sets of a dielectric coaxialresonator 31 of λ/4 whose outer conductor 31a is grounded and asubstrate 33 having an amplifier circuit 45 as an active circuit whichis coupled to the dielectric coaxial resonator 31.

This means that this second band-pass filter 21 comprises, for example,a cuboid-shaped case 27 composed of metal, and space in this case 27 isdivided into eight housing parts by seven partition plates 29 composedof metal. Then, in each of these housing parts, for example, thedielectric coaxial resonator 31 of λ/4 and the substrate 33 composed ofan insulating material such as epoxy resin are housed. In this case, theouter conductor 31a as one end of each dielectric coaxial resonator 31is grounded to the case 27. Also, each substrate 33 is disposed at theopen end side of the dielectric coaxial resonator 31.

As shown in FIG. 8A, on this substrate 33, for example, an L-shapedconductor pattern 35 is formed, and to one end of this conductor pattern35, an inner conductor 31b as the other end of the dielectric coaxialresonator 31 is connected, for example, by a conductor ribbon 37.Furthermore, on the substrate 33, two electrodes 39a and 39b are formedin the vicinity of one end of the conductor pattern 35, and theseelectrodes 39a and 39b are coupled to the conductor pattern 35respectively through gap capacities produced by gaps 41a and 41b. Also,connected to these electrodes 39a and 39b, the amplifier circuit 45provides an active circuit which is configured, for example, with a Sibipolar transistor, the connection being by lines 43a and 43b for phaseadjustment. Accordingly, this amplifier circuit 45 is coupled to thedielectric coaxial resonator 31. Also, this amplifier circuit 45 isconfigured so as to become a negative resistance in the operation bandof the second band-pass filter 21. For this reason, the apparent valueof Q of the second band-pass filter 21 becomes large in the operationband of the second band-pass filter 21. Furthermore, on the substrate33, a conductor pattern 47 for supplying power to this amplifier circuit45 is formed.

Furthermore, the other end of the conductor pattern 35 on each substrate33 is connected to a respective electrode 51 of a transmission line, bya conductor ribbon 49. Also, these electrodes 51 are disposed in a linewith intervals kept between them so that gap capacities are produced bygaps 52 between them. Accordingly, an equivalent circuit of thisdielectric coaxial resonator 31 and the surrounding components is formedas shown in FIG. 8B.

Furthermore, the electrodes 51 at the both ends are coupled to the inputend 21a and the output 21b, for example, through gap capacities producesby gaps 54. Accordingly, this second band-pass filter 21 is expressed byan equivalent circuit as shown in FIG. 9. The output end 21b of thissecond band-pass filter 21 is put in no-load state in this embodiment.In addition, to the output end 21b of this second band-pass filter 21, aresistive load, for example, may be connected.

Also, the dielectric coaxial resonator 31 and the gap capacities such asthose between the electrodes 51 in the second band-pass filter 21 areselected so that the resonance frequencies thereof are located in theband of 845-846.5 MHz to be attenuated. Accordingly, for this secondband-pass filter 21, the frequency characteristic of reflection at theinput end 21a side becomes as shown in FIG. 10.

In addition, though not illustrated, the conductor pattern 47 of eachsubstrate 33 is connected to a power source end 21c penetrating throughthe side wall of the case 27. Then, to this power source end 21c, the DCpower source 48 in the case 12 is connected, and power is supplied fromthe DC power source 48.

Another second band-pass filter 23 has a structure similar to theabove-described band-pass filter 21, but differs therefrom in that thishas ten dielectric coaxial resonator stages and the like and the passband is the band of 849 MHz and higher.

Still another second band-pass filter 25 differs from theabove-described band-pass filter 21 in that this has dielectric coaxialresonator stages and the like, and the pass band is the band of 835 MHzand lower.

For these second band-pass filters 23 and 25, the output ends thereofare also put in no-load state, and power is supplied from the DC powersource 48 also to power source ends 23c and 25c of these secondband-pass filters 23 and 25. In addition, loads, for example,resistances, may be connected also to the output ends of these secondband-pass filters 23 and 25.

In addition, the amplifier circuit in each second band-pass filter isconfigured so as to become a negative resistance in the operation bandof the second band-pass filter.

A third terminal 19c of the circulator 19, which becomes the output sidewhen the second terminal 19b of the circulator 19 is the input side, isconnected to the coaxial connector 50 as the output terminal penetratingthrough the side wall of the case 12.

In this filter apparatus 10, the first band-pass filter 16 transmits asignal of the band containing the desired frequency bands to the firstend 19a of the circulator 19. Then, this circulator 19 gives the signalto the input ends 21a, 23a and 25a of three second band-pass filters 21,23 and 25.

One second band-pass filter 21 has the pass band in the frequency bandof 845-846.5 MHz to be attenuated, and band, giving them to the secondterminal 19b of the circulator 19. Similarly, the other second band-passfilter 23 sharply reflects signals of bands other than the band of 849MHz and higher, and gives them to the second terminal 19b of thecirculator 19, and the still other second band-pass filter 25 sharplyreflects signals of bands other than the band of 835 MHz and lower,giving them to the second terminal 19b of the circulator 19.Accordingly, the signal of the desired frequency bands is given to thesecond terminal 19b of the circulator 19. For this reason, the signal ofthe desired frequency bands is outputted from the third terminal 19c ofthe circulator 19, and the signal is given to the coaxial connector 50as the output terminal. Accordingly, in this filter apparatus 10, afrequency characteristic as shown in FIG. 11 is obtained.

In the embodiment as shown in FIG. 7, the first band-pass filter 16 isinstalled as the pre-stage of the circulator 19, as shown in FIG. 16, itis also possible for no filter to be installed as the pre-stage of thecirculator 19. In this case, the first terminal 19a of the circulator 19is connected directly to the coaxial connector 14 as the input terminal.

Also, in the embodiment as shown in FIG. 7, dielectric coaxialresonators of λ/4 are used in the second band-pass filter, butdielectric coaxial resonators of λ/2 may be used in place of λ/4. Inthis case, as shown in FIG. 12 and FIG. 13, one end side of the innerconductor 31b of the dielectric coaxial resonator 31 of λ/2 is coupledto the amplifier circuit 45 as an active circuit, and the other end sidethereof is connected to the electrode 50 as a transmission line. Or, forthe resonator used in the second band-pass filter, a resonator havinganother structure such as the strip-line resonator may be used withoutbeing limited to the dielectric coaxial resonator, and the resonancemode thereof may be a mode such as the TE mode or the TM mode.

Furthermore, the embodiment as shown in FIG. 7 has a characteristic ofattenuating a plurality of bands, but the present invention isapplicable also to a filter apparatus having a characteristic ofattenuating one band. In this case, for example, in the embodiment asshown in FIG. 7, only the input end of one second band-pass filterhaving the pass band in the band to be attenuated has to be connected tothe second terminal 19b of the circulator 19.

Also, it is possible for an active circuit to be coupled to only one ofthe resonators. In this case, the active circuit is desirably coupledonly to the pre-stage or post-stage resonator, and in view of noisegenerated on the circulator side from the second band-pass filter, it ismost desirable to couple the active circuit to the post-stage resonator.

In addition, each of the embodiments as shown in FIG. 1 and FIG. 7 has acharacteristic of transmitting two different bands, but in the presentinvention, a filter apparatus having a characteristic of transmittingthree or more different bands may be configured. In this case, forexample, in the embodiment as shown in FIG. 1, three or more band-stopfilters having attenuation bands differing from one another have only tobe installed in the post-stage of the band-pass filter, or, in theembodiment as shown in FIG. 7, three or more second band-pass filtershaving pass bands differing from one another have only to be connectedto the circulator.

Although embodiments of the present invention have been described andillustrated in detail, it is clearly understood that the same is by wayof illustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A filter apparatus wherein a desired frequencyband is located in a pass band thereof and undesirable frequency bandsare located in attenuation bands thereof, comprisinga case, an inputterminal fixed to one side wall of said case, a band-pass filter in saidcase, an input end of which is connected to said input terminal, andwhich is for transmitting a band containing said pass band, a pluralityof band-stop filters in said case which are connected in series as thepost-stage to said band-pass filter, and are for attenuating saidattenuation bands, and an output terminal fixed to said one side wall ofsaid case, and connected as the post-stage to a plurality of saidband-stop filters,wherein at least one said band-stop filter comprises aresonator, and an active circuit which is coupled to said resonator, andbecomes a negative resistance in an operation band of the band-stopfilter.
 2. A filter apparatus in accordance with claim 1, wherein saidresonator includes a dielectric coaxial resonator.
 3. A filter apparatuswherein a desired frequency band is located in a pass band thereof andundesirable frequency bands are located in attenuation bands thereof,comprisingan input terminal, a first band-pass filter, an input end ofwhich is connected to said input terminal, and which is for transmittinga band containing said pass band, a non-reversible circuit element, afirst terminal of which is connected to an output end of said firstband-pass filter, a plurality of second band-pass filters, input ends ofwhich are connected in common to a second terminal of saidnon-reversible circuit element which is the output side when said firstterminal is set as the input side, and pass bands of which are locatedin said attenuation bands, and an output terminal which is connected toa third terminal of said non-reversible circuit element which is theoutput side when said second terminal is set as the input side,whereinat least one said second band-pass filter comprises a resonator, and anactive circuit which is coupled to said resonator, and becomes anegative resistance in an operation band of the second band-pass filter.4. A filter apparatus in accordance with claim 3, wherein said resonatorincludes a dielectric coaxial resonator.
 5. A filter apparatus inaccordance with claim 3, wherein an output end of said second band-passfilter is put in no-load state.
 6. A filter apparatus in accordance withclaim 3, wherein said second band-pass filter includes a plurality ofsaid resonators, and said active circuit is coupled to a last-stageresonator of said band-pass filter.